1. Field of the Invention
This invention relates to a process for the production of thixotropic agents, especially for reactive polyurethane systems, by reacting polyisocyanates containing in particular uretdione groups with polybasic carboxylic acids containing at least one carboxyl group and, optionally, OH groups, preferably in the presence of polyether polyols of relatively high molecular weight. These reaction products, preferably in polyether polyols, are particularly suitable for use as thixotropic agents in the production of coating compositions and adhesives from storable reactive polyurethane systems, so-called one-component systems.
2. Description of the Prior Art
It is known that coating compositions and adhesives which are not intended for application to horizontal surfaces have to show certain flow properties to ensure that they do not run during hardening. To this end, various thixotropic aids are added to prevent not only running, but also the sedimentation of fillers, pigments or other solid particles such as finely divided solid reactants. Thus, inorganic thixotropic and thickening agents are known to include finely divided pyrogenic (modified) silica and modified layer silicates (=smectites), while organic thixotropic and thickening agents are known to include castor oil derivatives.
In the production of coating compositions or adhesives based on polyurethanes, which are not intended for application to horizontal surfaces, finely divided polyureas of aliphatic or aromatic polyamines and aliphatic or aromatic polyisocyanates have previously been used as thixotropic agents. These polyureas may be added either to the polyol component or to the polyisocyanate component and bring about effective thickening of the corresponding component, even before application. If this is undesirable for reasons of practical application, the thixotropic effect may also be developed during application, i.e. in the mix head, enroute to the mold or directly on the surface to be coated, by initially adding the polyamine to the polyol component and then rapidly developing the thixotropic effect by addition of the corresponding isocyanate component with formation of polyurea.
Even in the case of the known, thermosetting, storable reactive polyurethane systems, so-called one-component systems (cf. DE-OS No. 32 30 757 or DE-OS No. 31 12 054), which may be hardened inter alia by the application of heat to form polyurethane, it may be necessary to add the thixotropic agent to the reactive mixtures not only to prevent the coating compositions or adhesives applied from running, but also to prevent sedimentation of the suspended solid constituents during storage.
The known thixotropic agents are not satisfactory because their effect when used in the usual quantities is generally too weak. However, if the desired effect is obtained by increasing the quantity added, the fully reacted polyurethane shows inadequacies, such as reductions in strength when inorganic thixotropicizing agents are used or defects through bubble formation or a reduction in adhesive effect when organic thixotropic agents, such as polyureas, are added. Polyureas are attended by the further disadvantage that the desired anti-sedimentation effect is not obtained in the event of long-term storage.
Accordingly, an object of the present invention is to provide a suitable thixotropic and thickening agent for thermosetting, storable reactive polyurethane systems (so-called one-component polyurethane systems), which is not attended by any of the known disadvantages.